An OLED (Organic Light Emitting Diode) display device is proposed as a next generation flat panel display device due to its advantages of self-illumination, high contrast, small thickness, wide viewing angle, fast response speed, flexibility, wide operating temperature range, simple structure and simple fabricating process, etc.
However, the OLED display device has limitations in high resolution and high color gamut. In the OLED display device, a threshold voltage of the TFT (Thin Film Transistor) driving the OLED to emit light is related to many factors, comprising doping of a drain of the TFT, a thickness of a dielectric, material of a gate, excess electric charges in the dielectric, etc. However, during fabricating a current display panel, especially a large-sized display panel, the consistency of such factors can hardly be achieved due to limitations of process conditions and technological level, which causes each TFT to have a different deviation in the threshold voltage, and further, long-time operating will also lead to lowered stability of the TFT. Therefore, to ensure the uniformity of the brightness of the OLED display panel, an additional compensation circuit is needed to compensate the inconsistency (or deviation) of the threshold voltages of the TFTs, however, most of compensation circuits need 4-6 TFTs and 1-2 storage capacitors, which usually renders a large pixel size and then does not help development of high resolution.